1. Field of the Invention
The invention relates to frequency deviation measuring devices which measure frequency deviations with respect to carrier frequencies of digital modulation signals.
2. Prior Art
The frequency deviation measuring devices are used for frequency measuring devices to perform measurement on carrier frequencies of digital modulation signals. Specifically, the frequency deviation measuring devices use the phase locus method to measure frequency deviations of QPSK modulation signals of .pi./4 shift.
It is well known that the .pi./4 QPSK modulation method is employed in fields of digital mobile communication systems such as digital automobile phone systems, second-generation cordless phone systems. In those systems, great errors should occur if a frequency counter is used to measure a frequency. For this reason, the conventional systems employ the frequency deviation measuring method using the phase locus method.
FIG. 5 shows an example of a frequency deviation measuring device utilizing the conventional frequency deviation measuring method.
In FIG. 5, a signal generator 11 inputs a testing frequency set value ft to generate a reference signal S11. Herein, the reference signal S11 coherently corresponds to the set value ft. A quasi-synchronization wave detector 12 performs synchronous wave detection on a testing signal Sin to produce an IQ base band signal S12. Herein, the testing signal Sin is subjected to .pi./4 shift QPSK modulation based on the reference signal S11 generated by the signal generator 11. A frequency deviation detector 13 effects the phase locus method on the IQ base band signal S12, produced by the quasi-synchronization wave detector 12, to detect a frequency deviation S13 which is measured on the basis of the testing frequency set value ft.
Next, the content of the phase locus method will be described in detail with reference to FIGS. 6 and 7.
As shown in FIG. 6, the .pi./4 shift QPSK modulation method determines 4 series of values (or coordinates)(i.e., 00, 01, 11, 10) based on a phase variation regarding a certain start point `0` so as to contribute to data transmission.
FIG. 7 shows the content of the phase locus method on the basis of the assumption that a movement occurs from the start point 0 to a point of coordinates (00), for example. Herein, the movement brings an actual point `A` other than an ideal point `B`. So, a phase deviation .DELTA..theta. is detected between the actual point A and the ideal point B. Thus, the phase locus method converts the phase deviation .DELTA..theta. to a frequency deviation.
In the conventional frequency deviation measuring device described above, however, a range of measurement should be limited to frequencies approximately corresponding to 1/10 of a symbol rate. This is because if a frequency error is great, an error may occur in a decision of a symbol so that an accurate frequency deviation cannot be obtained.
For example, a transmission of a symbol of `01` requires an amount of phase shift of 135.degree.. If a frequency error of `+4` kHz is added to the above amount of phase shift, in a system having a transmission speed of 21 kHz symbol/s, an amount of phase shift of 68.degree. should be added, so that the measuring device will obtain a total amount of phase shift of 203.degree..
As described above, the transmission of the symbol of `01` has an ideal amount of phase shift of 135.degree., whilst a transmission of a symbol of `11` has an ideal amount of phase shift of 225.degree.. So, the aforementioned total amount of phase shift is close to the ideal amount of phase shift provided for the transmission of the symbol of `11`, rather than the ideal amount of phase shift provided for the transmission of the symbol of `01`. As a result, the measuring device mistakenly detects that the transmission of the symbol of `11` is made on the basis of the total amount of phase shift which should correspond to the transmission of the symbol of `01`.
When measuring a frequency error with respect to a symbol point `11` by using the phase locus method, the device performs a calculation to subtract an ideal amount of phase shift from an actual amount of phase shift for `11`, as follows: EQU 203.degree.-225.degree.=-220.degree.
That is, the device calculates a frequency error of -1.3 kHz. This causes a difference against the testing signal Sin which is actually shifted by `+4` kHz.
The basic standards (RCR STD-27) for the digital automobile phone systems allow frequency deviations of approximately `.+-.3` kHz with respect to a symbol rate of 21 kHz symbol/s. So, the standards allow the measuring devices to have ranges of measurement of `.+-.4` kHz or more, i.e., ranges of measurement corresponding to 1/5 or more of the symbol rate.